1. Field of the Invention
The present invention relates to a portable radio device removably fitted to a mobile body, more particularly, to a portable radio device fitted to a mobile body which is a human body.
2. Description of the Related Art
In a mobile radio communication system, a radio device, such as a radio-paging device (pager) or a portable telephone, which is of a portable type and is not fixed to a mobile body, has been popularized in recent years, besides a communication device fixedly provided in a mobile body such as a vehicle. Furthermore, a portable radio device is used not only for ringing or voice communication but also for various data transmission.
Generally, in a mobile radio communication system, the communication quality is largely affected by interference due to the multiple radio wave paths or fading associated with the movement of a mobile body and so on, and then various countermeasures have been disclosed for suppressing such interference.
For example, in "Mobile Radio Reception System" disclosed in Japanese Patent Application Laid-Open No. 58-46892 (JP 58-46892/1983), three or more directional antennas, each having directivity within a limited angular range in a horizontal plane, are disposed such that each antenna forms an antenna directivity pattern in a respectively different directions and as a whole to be nondirectional. The signal strength received from each of these antennas is compared with a predetermined threshold to switch the output from each antenna, thereby performing diversity reception. In the application of this system, the antenna are necessarily integrated with a receiver. A radio device having antennas fixedly mounted to a mobile body such as a vehicle is the most appropriate.
Additionally, "Portable Radio Device" disclosed in Japanese Patent Application Laid-Open No. 3-6927 (JP36927/1991) comprises an antenna and components having a removable metal portion, such as a battery, and a single housing. For the purpose of suppressing the affect of the mounting state of those components on the antenna characteristics, the portable radio device is provided with determining means for determining the mounting state of the components and control means for controlling various constants which define the characteristics of the antenna in accordance with the determined mounting state of the components.
On the other hand, a small device among the above-mentioned portable radio devices, for example a portable radio receiver for broadcasting or a portable telephone, typically has a single antenna provided inside or attached outside a housing of the communication device. While moving time along, the entire communication device is used while being fitted to a human body by putting the entire communication device including the antenna in a bag or a pocket of clothing, or by holding the device in a holding case attached to a belt.
When the portable radio device is fitted to the human body as described above, typically it is not specified which surface of the housing of the portable radio device is disposed closely to the human body. If the surface of the housing on which the antenna is disposed is close to the human body, the directivity of the antenna may be disturbed by the human body to deteriorate the reception sensitivity and the transmission incident power.